This invention relates to oriented thermoplastic films for packaging applications, and more particularly relates to coextruded multilayer, oriented film having good strength and oxygen barrier characteristics.
Thermoplastic film, and especially polyolefin materials, have been used in the past to package various articles including perishable food products which require protection from the environment, resistance to physical and environmental abuse during storage and distribution, and an aesthetic and attractive appearance. Optical properties such as high gloss, high clarity, and low haze contribute to the aesthetic consumer appeal of products wrapped in such packaging materials. Good optical properties also permit adequate inspection of the packaged product during the distribution cycle and by the end-user at point of purchase.
Oxygen barrier characteristics are required to provide extended shelf life for the packaged product in the case of food items, and various materials has been used to provide lower gas permeability and thereby reduce the oxygen transmission of the film. For example, ethylene vinyl alcohol copolymer (EVOH) has been known for some time as a good oxygen barrier material, and has been used in the past in conjunction with multilayer packaging films, as described below in several patent publications. EVOH has also proved to be a good barrier to odors or fragrances.
A shrink feature may be imparted to a thermoplastic film by orientation of the film during its manufacture. This shrink feature allows the film to shrink or, if restrained, create shrink tension within the packaging film upon exposure to heat, for example, in a hot water bath or by exposure to hot air. In a typical process, manufactured film is stretched in either the machine direction or perpendicular to the machine direction, or both, i.e., in the longitudinal and transverse directions respectively, in varying degrees to impart a desired degree of shrinkability to the film upon subsequent heating. After this stretching operation, the film is rapidly cooled to impart this latent shrinkability to the resulting film. Shrinkable film provides a tight, smooth appearance to a product wrapped in such film, as well as some added toughness in order to protect the packaged product from abuse.
Of interest is U.S. Pat. No. 4,421,823 issued to Theisen et al directed to a flexible wrapping material of limited construction having a biaxially oriented polypropylene/oxygen barrier substrate, in which the oxygen barrier material may be EVOH; an extrusion laminate of a biaxially oriented polymer such as polypropylene or nylon, bonded to polyethylene; and a layer of heat sealable polymeric material such as ethylene vinyl acetate copolymer laminated to the substrate. A special polymer which may be, for example, polyethylene or ethylene vinyl acetate copolymer is bonded to one surface of the biaxially oriented polypropylene.
Of interest is U.S. Pat. No. 4,457,960 issued to Newsome, disclosing a multilayer film having a core layer of a barrier material such as EVOH and EVOH blends. This film may be shrinkable and may be melt extruded, and may contain outside layers having a blend of linear low density polyethylene and ethylene vinyl acetate copolymer.
Of interest is U.S. Pat. No. 4,464,443 issued to Farrell et al, disclosing the use of EVOH in a five layer structure having outside layers of high density polyethylene and intermediate layers of Plexar adhesive.
Of interest is U.S. Pat. No. 4,495,249 issued to Ohya et al, disclosing a five-layer film having a core layer of saponified copolymer of ethylene and vinyl acetate, two outer layers of a mixture of linear low density polyethylene and ethylene vinyl acetate copolymer, and two adhesive layers disposed between the core layer and outer layers.
Of interest is U.S. Pat. No. 4,514,465 issued to Schoenberg, disclosing a five-layer thermoplastic film having surface layers comprising a four-component blend of linear low density polyethylene, linear medium density polyethylene, ethylene vinyl acetate copolymer and at least one ultra-violet light stabilizer.
Of interest is U.S. Pat. No. 4,398,635 issued to Hirt and disclosing a medication package in which a coextruded multiple layer sheet may have a structure including a layer of ethylene vinyl alcohol copolymer sandwiched between adjacent layers of nylon, and in which one of the nylon layers may be further adhered to a tie resin. The nylon layers may form either an outside surface or, in one example, internal layers with additional layers of polymeric materials added to each side of the sandwich structure.
Of interest is U.S. Pat. No. 4,355,721 issued to Knott et al and disclosing a coextruded multilayer sheet having a first layer of nylon, an EVOH barrier layer, another layer of nylon, an adhesive layer, and another outside layer of, for example, high density polyethylene.
Of interest is U.S. Pat. No. 4,284,674 issued to Sheptak and disclosing a multilayer film having a core layer of ethylene vinyl alcohol copolymer adhered on each side to nylon, each nylon layer in turn being adhered to a chemically modified polyolefin, and a further layer of primer material suitable to adher the modified polyolefin to an outer layer of polypropylene or other materials suitable for conveying toughness, flexcrack resistance and moisture barrier properties to the multi-ply film.
U.S. Pat. No. 4,407,873 issued to Christensen et al, discloses a packaging material for retort applications including a heat seal layer of linear low density polyethylene, a second layer of linear low density polyethylene with optionally 0% to 80% medium density polyethylene blended into the second layer, a third layer of anydride modified medium density polyethylene, a fourth layer of nylon, a fifth layer of ethylene vinyl alcohol copolymer, and a sixth layer of nylon.
Of interest is U.S. Pat. No. 4,400,428 issued to Rosenthal et al, which discloses a composite film having a biaxially oriented polypropylene base film (BOPP) laminated on at least one surface with a multlayer structure including a gas barrier layer of a hydrolyzed ethylene vinyl acetate copolymer and a layer adjacent to the base film, and a heat sealable outer layer which may be, for example, modified propylene/ethylene copolymer. Adhesion-promoting layers of modified polyolefin may include polypropylene containing grafted units of alpha, betamonounsaturated dicarboxylic acids.
U.S. Pat. No. 4,501,797 issued to Super et al, discloses an unbalanced oriented multiple layer film including a first layer of polypropylene, a second layer of an anhydride modified polypropylene, and a third layer of ethylene vinyl alcohol copolymer.
Of interest is U.S. Pat. No. 4,501,798 issued to Koschak et al, disclosing the use of a blend of EVOH and nylon in an unbalanced multiple layer polymer film, also including either linear low density polyethylene or ethylene vinyl acetate copolymer in a sealant layer. Adhesive layers of preferably anhydride derivatives are also present.
It is, therefore, an object of the present invention to provide a coextruded thermoplastic multilayer film characterized by good oxygen barrier properties.
It is a further object of the present invention to provide a thermoplastic multilayer film having an aesthetically pleasing appearance with good optical properties.
It is another object of the present invention to provide a relatively thin thermoplastic multilayer film having superior toughness and abrasion resistance.
It is still another object of the present invention to provide a coextruded thermoplastic multilayer film which may be totally coextruded and then oriented to provide a shrinkable film with good oxygen barrier properties.